1. Field of the Invention
The present invention relates to a functional reinforcing filler including inorganic particles having a surface modified by an organic silanepolyol obtained by hydrolyzing an organic alkoxy silane compound and a method of preparing the same.
2. Description of the Related Art
In the production of tires, it is common to use various additives to improve the physical properties of an organic polymer. In particular, silica has been widely used in recent years as a reinforcing filler in rubber used to produce tires. A rubber composition including silica may improve tensile strength, tear resistance and abrasion resistance of tires, and may also reduce rolling resistance and improve wet traction. However, a silica filler, as a sole reinforcing filler, may not be well dispersed in rubber and coupled in the rubber due to hydrophilic groups on the surface thereof, so as not to improve physical properties of rubber. In order to overcome this problem, a variety of silane coupling agents have been used with the silica filler.
In order to use a silane coupling agent, a silane coupling agent may be added to a rubber composition, or the surface of a silica filler may be modified using a silane coupling agent. For this, an organic alkoxy silane containing sulfur such as mercaptoalkyltrialkoxysilane and bis(alkoxysilylalkyl)polysulfide has been used in the tire industry.
When a mercaptoalkyltrialkoxysilane coupling agent is used with silica, physical properties of the rubber may be improved. However, the mercaptoalkyltrialkoxysilane coupling agent causes irritating odors, and time and energy to mix them with a polymer composition increase, processibility of the rubber may be reduced due to too short prevulcanization time.
In the early 1990's, green tire using bis(triethoxysilylpropyl)tetrasulfide (TESPT) was reported by Michelin Tire Corp. [Eur. Patent EP0501227, U.S. Pat. No. 5,227,425]. Since then, TESPT has been widely used to crosslink and cure silica and a synthesis rubber in tires. However, the preparation of rubber using a silica filler and TESPT needs to be performed within a narrow temperature range. The temperature for mixing silica and rubber should be high enough to permit the reaction between silica and an alkoxy silane coupling agent to take place rapidly, since an alkoxy group of the alkoxy silane coupling agent reacts with a hydroxyl group (—OH) of silica at a very high temperature. However, when using TESPT, irreversible thermal degradation of a polysulfane functional group and prevulcanization of a rubber mixture are inevitable at a high temperature. On the other hand, at a low temperature, alkoxy silane is too slowly hydrolyzed, so that unreacted alkoxy groups remain and can be available to further hydrolysis to generate alcohol during subsequent stages while mixing the rubber composition. The alcohol remaining around the rubber matrix is emitted to cause environmental problems. This may form pores or bubbles on the surface of rubber products, thereby causing defects on tire treads. In addition, it is known that TESPT cannot be completely hydrolyzed.
Wolff, S reported that up to ⅔ of alkoxy groups can be hydrolyzed due to steric hindrance of TESPT [Wolff, S. Kautsch. Cummi, Kunstst 1981, 34, 280]. The hydrolysis of unreacted alkoxy groups can continue through the life of a product manufactured from an elastomer compound to cause volatile organic compounds. In addition, due to incomplete hydrolysis, the TESPT is less coupled to silica.
Therefore, diverse research is being conducted into a modified silica or a rubber composition having minimized alkoxy groups by complete hydrolysis of an organic silane coupling agent.